Figuring out the type of `window.Jupyter` in a classic notebook

I’m the author of a JupyterLab extension, and I’m in the middle of porting it to the classic notebooks. Doing so is fairly straight forward - but I’m relying on apis provided under window.Jupyter.notebook.

Since my extension is a typescript project, I’m looking for the type definitions for window.Jupyter and it’s subcomponents.

Does anyone know where I can find these? I’ve been trawling through documentation for a few hours, and no luck.

Thanks for any help!